1. Field of the Invention
The present invention relates to a technique to provide a small-size high-magnification and high-quality lens configuration as a zoom lens for a video camera for civil use.
2. Description of the Prior Art
As a conventional four-group inner focus zoom lens for civil video camera use having a comparatively high magnification of 6 times or above and including a minimum number of lenses in practice, there is a 9-lens zoom lens. That is, as shown in FIG. 1, the 9-lens zoom lens xe2x80x9caxe2x80x9d includes: a first lens group GR1 having a cemented lens consisting of a first lens L1 which is a concave meniscus lens and a second lens L2 which is a convex lens, and a third lens L3 which is a convex meniscus lens; a second lens group GR2 having a fourth lens L4 which is a concave meniscus lens, and a cemented lens consisting of a fifth lens L5 which is a concave lens and a sixth lens L6 which is a convex lens; a third lens group GR3 having a seventh lens L7 which is a single convex lens; and a fourth lens group GR4 having a cemented lens consisting of an eighth lens L8 which is a concave lens and a ninth lens L9 which is a convex lens.
In the aforementioned conventional zoom lens xe2x80x9caxe2x80x9d, the third lens group GR3 is not a so-called achromatic lens and a chromatic aberration generated in the third lens group GR3 has been compensated by excessively performing correction of chromatic aberration in the fourth lens group GR4.
However, there is a problem that aberration fluctuation such as a chromatic aberration and a spherical aberration of the first lens group GR1 generated at magnification can be canceled by the reverse aberration generated in the second lens group GR2, but bending by the chromatic aberration and spherical aberration generated by displacement of the fourth lens group GR4 cannot be canceled.
The lenses of the fourth group GR4 should be made from such a material that the a difference between an Abbe""s number of the eighth lens L8 which is concave lens and an Abbe""s number of the ninth length L9 which is a convex lens is as large as possible. Moreover, the junction surface curvature between the eighth lens L8 and the ninth lens L9 is also limited because the chromatic aberration is corrected into a range allowable for a wide angle end and the refracting power distribution between the eighth lens L8 and the ninth lens L9 is determined dependently from the chromatic aberration.
Accordingly, the bending by the color of the spherical aberration generated by the displacement of the fourth lens group GR4 is determined by the material of the eighth lens L8 and the ninth lens L9, the refracting power distribution, and the junction surface curvature. Thus, there has been almost no degree of freedom in correction.
The conventional zoom lens xe2x80x9caxe2x80x9d, a lens system consisting of nine lenses can be used in practice when the magnification is low, the size need not be small, and the F number may be dark, but cannot be used when a high magnification is required, the size should be small, and a high image quality is required.
For increasing the magnification of the zoom lens xe2x80x9caxe2x80x9d, when the entire refracting power distribution is performed so that the position of the fourth lens group GR4 is almost identical at the wider angle end and the telephoto end for an infinite point, then the displacement amount of the fourth lens group GR4 at the intermediate focal position becomes too great and the chromatic aberration fluctuation becomes remarkable and this chromatic aberration cannot be corrected because of the design limitation of the fourth lens group GR4 as has been described above.
Moreover, in order to make smaller the size of the zoom lens xe2x80x9caxe2x80x9d, it is advantageous each of the lenses has a strong refracting power and displacement of the displacable lens groups is small. However, when the second lens group GR2 has a strong refracting power, mainly the Petzval""s sum is a negative great value, causing excessive correction, which makes difficult correction of the image surface bending. When each of the lens groups has a strong refracting power, aberration such as spherical aberration, especially aberration generated by cancellation between the first lens group GR1 and the second lens group GR2 becomes strong and it becomes difficult to obtain a preferable performance in the entire zoom region.
To cope with this, conventionally, in order to reduce the spherical aberration generated in the first lens group, the refractive index of the third lens L3 which is a convex lens has been increased. However, in case of the refractive index of the third lens L3 which is a convex lens of the first group GR1, this makes the entire Petzval""s sum negative. That is, it is impossible to simultaneously obtain suppression of generation of aberration inherent to the first lens group GR1 and making the refracting power of each lens stronger to minimize the lens size.
Moreover, in the fourth lens group GR4 of the zoom lens xe2x80x9caxe2x80x9d, when an aspherical surface is to be realized by glass mold, it cab be formed only on the ninth lens which is a convex lens because of the molding easiness. For example, when forming a composite spherical surface using an ultraviolet ray hardening resin, it can be formed only on the surface of the ninth lens L9 because material used for the eighth lens L8 which is a concave lens does not pass ultraviolet rays. This limits the design and it has been impossible to increase the number of surfaces constituted by an aspherical surface which is effective for improving the image quality.
It is therefore an object of the present invention to provide a high-magnification, small-size, and high image quality lens configuration for the zoom lens used in a video camera of civil use.
In order to achieve the aforementioned object, the present invention provides a zoom lens comprising a first lens group having a positive refracting power and its position fixed, a second lens group having a negative refracting power and displacable mainly for magnification, a third lens group having a positive refracting power and its position fixed, and a fourth lens group having a positive refracting power and displacable mainly for correction of the focal position for magnification and focusing, wherein the first lens group includes, viewed from an object, a cemented lens consisting of a first lens which is a concave meniscus lens having a convex surface facing the object side and a second lens which is a convex lens, and a third lens which is a convex meniscus lens having a convex surface facing the object side; the second lens group, viewed from an object, includes a fourth lens which is a concave meniscus lens having a convex surface facing the object side, and a cemented lens consisting of a fifth lens which is a both-side-concave lens and a sixth lens which is a convex lens; the third lens group includes a seventh lens which is a convex lens; and the fourth lens group, viewed from an object, includes a cemented lens consisting of an eighth lens which is a convex lens having a convex surface facing the object side, a ninth lens which is a concave lens, and a tenth lens which is a convex lens, and wherein the third lens group includes at least one surface constituted by an aspherical surface and in the fourth lens group, at least the surface which is at the object side is constituted by an aspherical surface.
Accordingly, in the present invention, the refracting power of the ninth lens serving for achromatization in the third lens group and the fourth lens group is determined by the achromatic condition and has a characteristic similar to the ninth lens in the aforementioned conventional example. However, in the present invention, the ninth lens is sandwiched by convex lenses, which enables to obtain a significantly greater degree of freedom of the curvature of the ninth lens. In spite of that the tenth lens which is a convex lens has a convex surface facing the object side in the same way as in the conventional example, but its curvature can be designed more gentle than in the conventional example. This remarkably improves bending due to color of the spherical aberration generated from this surface.